1. Field of the Invention
The present invention relates to a circuit board for an ink jet head that ejects ink for printing, a method of manufacturing the circuit board, and an ink jet head using the circuit board.
2. Description of the Related Art
An ink jet printing system has a characteristic that it is possible to print a high-definition image at a high speed by ejecting a very small amount of ink out of a nozzle in the form of droplets, for example. There are a variety of ejection methods available for the ink jet printing head (hereinafter referred simply as a printing head when appropriate) to realize the ink jet printing system. Among them, ink jet heads using methods of ejecting ink by use of thermal energy have been disclosed in U.S. Pat. Nos. 4,723,129, 4,740,796, and the like. A head according to the foregoing methods includes an ink jet head circuit board provided with a plurality of heaters for heating ink and generating bubbles as well as wires and other constituents for establishing electrical connection to the heaters, all of which are fabricated on one substrate. Moreover, in a general configuration, nozzles for ejecting the ink are formed thereon in positions corresponding to the heaters. This configuration allows easy and high-precision production of ink jet head circuit boards incorporating high-density layouts of resistors, wires, and the like through a process similar to a semiconductor manufacturing process. Accordingly, this configuration can realize printing at higher resolution and higher speed. In addition, by using this configuration, it is possible to form a smaller ink jet head and eventually to form a smaller printer using such an ink jet head.
FIG. 1 is a schematic plan view showing a typical configuration of one heater and the vicinity thereof to be disposed on the printing head circuit board disclosed in the above-mentioned patent documents. In a circuit board 1100, an electrode wire layer 1105 is formed as an upper layer of a resistor layer 1104, and a heater 1104′ is formed by removing a portion of the electrode wire layer 1105 and exposing the resistor layer of that portion. An electrode wiring pattern is wired on the circuit board 1100 and is connected to a drive element circuit and an external power supply terminal. In this way, it is possible to receive power supply from outside. Here, the resistor layer 1104 is made of a material having a high electric resistance value. By applying a current from outside through the electrode wire layer 1105, the heater 1104′ corresponding to the portion without the electrode wire layer 1105 generates thermal energy and generates a bubble in ink.
FIG. 2 is a schematic cross-sectional view of the ink jet head circuit board 1100 taken along the II-II line in FIG. 1 in a position corresponding to a liquid passage. In FIG. 2, reference numeral 1101 denotes a substrate made of Si, reference numeral 1102 denotes a heat accumulating layer made of a thermally oxidized film, and reference numeral 1103 denotes an interlayer film incorporating a heat accumulating function which is made of a SiO film, a SiN film, or the like. The resistor layer 1104 is formed on the interlayer film 1103, and the electrode wire layer 1105 made of metal such as Al, Ai-Si or Al-Cu is further formed thereon. Moreover, the heater 1104′ is formed by partially removing the electrode wire layer 1105 and exposing the resistor layer of that portion. The electrode wire layer 1105 is wired in the circuit board 1100 and is connected to the drive element circuit and the external power supply terminal. In this way, it is possible to receive power supply from outside.
Further, a protective layer 1106 made of a SiO film, a SiN film, or the like is formed on the heater 1104′ and on the electrode wire layer 1105 for protecting and insulating these constituents from the ink. Moreover, a high-durability protective layer 1107 is formed thereon as a layer which can endure damages caused by, for example, chemical impacts or physical impacts associated with generation of a bubble in ink. A region of the high-durability protective layer 1107 above the heater 1104′, which contacts the ink, constitutes a heat applying portion 1108.
Incidentally, the ink jet printing system has been facing demands for printing performances in higher resolution, higher image quality, and higher speed in recent years along with its diffusion. Among them, one solution for the demands for higher resolution and higher image quality is to reduce an amount of ejected ink per dot (to reduce a diameter of an ink droplet when ejecting the ink as a droplet). Conventionally, the shape of the nozzle has been modified (or an area of an orifice is reduce) and an area of the heater has been reduced in order to obtain a smaller ink droplet.
Meanwhile, as a solution for the demand for higher speed printing, it is deemed effective to effectuate ejection of numerous ink droplets in a short time period either by raising a drive frequency while reducing a width of an electric pulse for driving an electrothermal transducer element or by increasing the number of nozzles for ejecting the ink. However, when applying this solution, the heat generated by the heaters is accumulated in the circuit board and the temperature of the head is raised. Therefore, a printing operation needs to be interrupted from time to time. Such interruption will pose a new problem of reduction in recording throughput.
Accordingly, there has been an attempt to devise an appropriate layer constitution so as to apply the heat generated by the heater to the ink efficiently for ink ejection.
For example, heat conductivity is increased more when a layer between the heater and a surface contacting the ink is thinner and the heat escaping to the side other than the ink is reduced. For this reason, it is possible to suppress the problems of the heat accumulation and the temperature rise of the printing head, and to reduce power consumption for generating the bubbles. That is, energy efficiency is improved as the effective thickness of the protective layer on the heater becomes thinner. On the contrary, if the protective layer is too thin, a pin hole existing on the protective layer may expose the heater or cause insufficient coverage of a step portion of a wire due to a failure to cover the step portion of the wire adequately. As a result, the ink may enter from that portion and cause corrosion of the wire or the heater, and eventually, may cause degradation in reliability and reduction in an operating life.
To deal with these problems, Japanese Patent Application Laid-open No. 8-112902 (1996) discloses a configuration in which first and second protective layers are provided and the first protective layer is removed at a heat applying portion. In this configuration, it is possible to improve energy efficiency and reduce power consumption, and to improve reliability and extend the operating life as the protective layers.
FIG. 3 is a schematic cross-sectional view of the heat applying portion and its vicinity of an ink jet head circuit board disclosed in Japanese Patent Application Laid-open No. 8-112902 (1996). In this configuration, a first protective layer 1106a and a second protective layer 1106b are formed on an electrode wire layer 1105, and the first protective layer 1106a constituting a lower layer is removed in a position above a heater 1104′. Specifically, the first protective layer 1106a made of a SiO film, a SiN film, or the like is formed firstly and then only the portion of the first protective layer 1106a corresponding to the heat applying portion 1108 is removed by patterning and the like. Thereafter, the second protective layer 1106b made of a SiO film, a SiN film, or the like is formed and then a high-durability protective layer 1107 is lastly formed thereon. By forming the substantially thin protective layer above the heat applying portion 1108, thermal energy from a resistor layer 1104 can be transferred to the ink only through the second protective layer 1106b and the high-durability protective layer 1107. For this reason, it is possible to use the thermal energy more effectively while obtaining given protecting and insulating function from the second protective layer 1106b. 
Here, in light of reducing an energy loss of the entire ink jet head system, it is effective to reduce a value of resistance of the electrode wires by increasing the thickness of the electrode wire layer. However, the increase in the thickness thereof means an increase in the level of the step portions to be formed by patterning. In consideration of the coverage of these step portions, it is inevitable to increase the thickness of the protective layers to some extent.
Now, in the configuration shown in FIG. 3, the first protective layer 1106a is removed at a site shifted from an end portion of the electrode wire layer 1105 facing a heater 1104′ to inside of the heater in order to sufficiently cover a step at the end portion. Here, although the entire surface of the heater generates heat, it is known that generation of bubble occurs only in a region (hereinafter referred to as an effective bubble generating region) excluding regions on the peripheries of the heater extending several micrometers inward because of an increase in the amount of the heat escaping from those peripheral portions of the heater. When adopting the configuration disclosed in Japanese Patent Application Laid-open No. 8-112902 (1996) described above, the first protective layer 1106a is removed at the site shifted from the end of the electrode wire layer 1105 facing the heater 1104′ to the inside of the heater. In other words, the first protective layer 1106a exists in the position shifted to the inside of the heater. For this reason, the actual effective bubble generating region is further limited and reduced, thereby degrading heat efficiency. That is, direct adoption of the technique disclosed in Japanese Patent Application Laid-open No. 8-112902 (1996) to the situation requiring a smaller area of the heater poses a problem of additional degradation of the heat efficiency.
Meanwhile, there is also an attempt to suppress a thermal energy loss generated at a wire or the like by increasing a value of resistance of a resistor while reducing a value of a current flowing on the entire ink jet head system. In terms of this request for higher resistance of the resistor, investigations for various materials and a method of reducing the thickness of the resistor have been attempted. Reduction in the thickness has a difficulty in satisfying various characteristics including manufacturing stability, characteristic stability, reliability, and the like. Nevertheless, there is a strong demand for satisfying the above-mentioned characteristics while reducing the thickness.
However, in the configuration shown in FIG. 3, the heater is formed by laminating the electrode wire layer on the resistor layer, patterning the electrode wire, and then patterning again after laminating the first protective layer 1106a. Accordingly, a surface of the resistor layer is affected by etching processes and the like applied to these patterning processes. That is, the surface is affected by plasma when dry etching is performed or is affected by an etchant when wet etching is performed. Moreover, the surface may be exposed to air in the course of delivery for conducting those etching processes. Due to these processes, the surface of the resistor layer may be oxidized or damaged by the etching processes, or absorb gas or water. As a result, variation and fluctuation in the values of resistance are sometimes observed. Moreover, reliability is sometimes degraded by partial oxidation or damages. As described above, it is difficult to satisfy the above-mentioned characteristics particularly when obtaining the configuration as shown in FIG. 3 while reducing the thickness of the resistor layer for a higher value of resistance. In addition, if there is a difference in a bubble generating phenomenon among the heaters due to the fluctuation of the values of resistance, it is not possible to ensure a given ink ejection amount in terms of each nozzle, and moreover, resulting in causing large fluctuation of the ink ejection amounts among the respective nozzles. This will lead to degradation in printing quality.